The present invention relates to a discharge-lamp lighting device for lighting a discharge lamp such as a metal halide lamp, and a system such as a liquid crystal projector using the discharge-lamp lighting device.
Recently, the brightness of projection-type liquid crystal projectors has been improved dramatically by the use of highly efficient high-pressure discharge lamps (hereinafter, may be abbreviated as xe2x80x98lamp(s)xe2x80x99). However, fluctuation in brightness on a screen, which is caused by a fluctuation of an arc track of a lamp, or a phenomenon called xe2x80x98arc jumpxe2x80x99, has been remarkable. A problem associated with this type of lamp is that the discharge arc becomes unstable depending on the temperature of the electrode and the condition of the surface of the electrode. As explained in U.S. Pat. No. 5,608,294, the discharge arc becomes unstable because the origin of the discharge arc jumps to some sharp-pointed projections (hereinafter, referred to as xe2x80x98spot(s)xe2x80x99) formed on the surface of the electrode.
The above-identified U.S. Pat. No. 5,608,294 discloses a method for solving the arc jump caused by the above factors. As shown in FIGS. 8 and 9, the method includes generating, by using either a bistable multivibrator or a flip-flop, a current pulse Pc in a predetermined fraction of given half periods of the lamp current, supplying a lamp current I obtained by superimposing a homopolar current to a back porch part of an alternating current waveform at a ratio of 0.05-0.15 to the half periods with an amount of current corresponding to 5-15% of the energy supplied to the lamp.
An effect of this method is the prevention of arc jump, which is obtained by preheating the electrode which was an origin of the arc at the time of a polarity inversion of the lamp current with a previous current pulse before the surface temperature of the electrode is lowered, so that a returning point of the arc after the polarity inversion matches with the previous origin of the arc. However, this method alone is insufficient to prevent the arc jump since the method does not addresses some problems including: spot formation that is caused by tungsten halide oxide and that have been remarkable in the recent trend of highly efficient lamps, lowering temperature of the electrode surface caused by the polarity inversion time, furthermore, matters of heat capacity of the lamp electrodes and bulb shapes, aging of the lamp, and a new method of controlling switching power fed to an identical lamp.
A problem concerning the lamp lighting is that, depending on the temperature and a surface condition of the electrode, the discharge arc becomes unstable. As described in U.S. Pat. No. 5,608,294, the instability of the discharge arc tube is caused by jumping of the origin of the discharge arc to some spots formed on the surface of the electrode. The arc jumps occur at the time of the polarity inversion of the lamp current. The reason is that the lamp current is zero-crossed inevitably at the moment of the polarity inversion of the lamp current, whereby a considerable lowering of the electrode surface temperature through the entire periods of the lamp current results.
For lighting a lamp, glow discharge is carried out by applying a high-voltage pulse between the electrodes in order to warm the electrodes gradually, and subsequently, the operation shifts to discharge thermoelectrons, so that the arc will discharge continuously. At that time, the arc origin tends to discharge electrons by selecting electrode spots on the electrode surface, since more electrons will spring out from such spots. The spots are formed, since a metal such as tungsten used as a material of the electrodes is heated to approximately the melting point during the lighting of the lamp, on places where electrons impinge to cause sputtering of the electrode and to deform the electrode slightly.
Moreover, since lamps have bulbs of small diameters in the recent trend for highly efficient lamps, the arc and the quartz glass forming the bulb get much closer to each other. As a result, halogenated tungsten combined with a halide filled in the bulb will be coupled easily with oxygen as a component of the quartz glass, increasing the generation of tungsten halide oxide. The tungsten, which is evaporated and combined with a halogen, is combined also with oxygen substances such as oxides adhering to a molybdenum foil or the like that composes a conductor of the electrodes, thereby forming tungsten halide oxide. Furthermore, since the tungsten halide oxide tends to be separated at a part with a comparatively high temperature, the evaporated tungsten that has been reduced at a part with a comparatively low temperature among the entire electrodes, is reduced intensively at spots as origins of arc at the tips of the electrodes, and further accelerates the growth of the spots.
The grown spot is spattered by an inrush current at the next lighting of the lamp, and thus, many small spots are formed on the electrode surface. While the lamp is lighting, fluctuation of the arc origin is generated on the electrode surface at some parts at which the tungsten halide oxide is reduced, thereby forming many spots. Considering this, the origins of discharge arc can be fixed and stabilized by decreasing considerably the number of the electrode spots and maintaining the fewer spots to have a sharp-pointed shape, where only one of the spots has a high temperature.
The present invention aims to solve the above-described problems, and an object thereof is to provide a discharge-lamp lighting device with fewer arc jumps and less fluctuation in brightness of a lamp even when an inexpensive, small and highly efficient lamp is used. This object can be achieved by accelerating positively the growth of spots. And a lamp of the present invention has a long life. The present invention provides also a system such as a liquid crystal projector using the discharge-lamp lighting device.
For achieving the above-described objects, a discharge-lamp lighting device according to the present invention comprises: a DC-DC converter for stepping down an input DC voltage in accordance with a current-controlling signal and outputting a desired current; a commutator for commutating a direct current from the DC-DC converter to an alternating current in accordance with a drive-controlling signal with a rectangular waveform; a high-pressure discharge lamp to be fed with the alternating current from the commutator; and a controlling portion for outputting the drive-controlling signal and also for outputting the current-controlling signal on the basis of a value of current flowing in the high-pressure discharge lamp or a value of voltage of the high-pressure discharge lamp in order to make electric energy in the high-pressure discharge lamp constant. The controlling portion sets a frequency of the drive-controlling signal to be within a predetermined frequency range so that sharp-pointed projections (spots) formed by an arc discharge on electrodes composing the high-pressure discharge lamp grow through a cycle of oxidation-reduction of a metal composing the electrode, superimposes a triangular wave signal generated on the basis of the drive-controlling signal onto the current-controlling signal through the entire periods of the drive-controlling signal in order to make a peak value of the current flowing in the high-pressure discharge lamp constant, and adjusts a waveform of the current flowing in the high-pressure discharge lamp.
According to this configuration, the polarity inversion frequency of the lamp current as a frequency of the drive-controlling signal is set to a frequency (e.g., 170 Hz) allowing growth of the spots and the waveform of the current amount can be adjusted to have a constant peak value of the back porch of the lamp current. Thereby, it is possible to preheat gradually the origin of the arc just before polarity inversion of the lamp current, suppress lowering of the electrode surface temperature at the time of polarity inversion, and then warm rapidly the arc origin of an opposite electrode soon after the polarity inversion. Therefore, a long life of the lamp can be realized by minimizing the fluctuation in temperature of the electrodes before and after the polarity inversion and accelerating growth with certain spots of high temperature.
That is, by accelerating the growth of certain spots and by decreasing a change in temperature at the arc""s origin and returning point at the time of polarity inversion of the lamp current, the spots with high temperature on the electrode surface can be fixed to avoid arc jumps. Furthermore, since one effect of the adjustment of the lamp current also can yield expectations of a long life of the lamp electrodes in that the electrode temperature during a discharge is apparently suppressed to be substantially constant while the current amount of the lamp varies, the obtained effect of decreasing the arc jump is superior to that reported in U.S. Pat. No. 5,608,294.
In the discharge-lamp lighting device according to the present invention, it is preferable that the controlling portion changes the triangular wave signal in at least one of the amount and the timing for superimposing onto the current-controlling signal in accordance with either the current value or the voltage value of the lamp.
A consideration of this structure is that substantially all the discharge-lamp lighting devices are controlled to provide a constant power to the lamp and furthermore, the voltage between the electrodes of the lamp is raised with use of the lamp. Namely, the lamp current will be decreased with aging of the lamp.
Specifically, the lamp current is decreased and the electrode temperature stops a remarkable rising over the working time of the lamp, and it will be difficult to discriminate discharging spots and non-discharging spots based on the temperature difference. Occurrence of arc jumps caused by aging of the lamp can be prevented if it is possible to adjust the amount and timing for superimposing the triangular wave signal onto the current-controlling signal inputted into the DC-DC converter in accordance with either the current or voltage of the lamp. For example, when the lamp current is decreased and the electrode temperature is not raised remarkably due to the change in the lamp voltage over time, occurrence of arc jumps can be prevented by increasing the amount of superimposing triangular wave signal and widening intentionally the temperature difference between discharging spots and non-discharging spots.
In the discharge-lamp lighting device according to the present invention, it is also preferable that the controlling portion changes at least one of the amount and the timing for superimposing the triangular wave signal onto the current-controlling signal in accordance with the temperature of the high-pressure discharge lamp.
This structure serves to prevent arc jumps that occur when the temperature difference between discharging spots and non-discharging spots is decreased due to variation in heat capacities of the lamps, specifically, differences in electrode structures and diameters, lamp bulb diameters, and cooling conditions of the lamps, or the environmental temperatures in use of the lamp. For example, when the working temperature of the lamp is low and the temperature of the lamp electrode is not raised sufficiently, superimposition of triangular wave signals can be increased and the temperature difference between discharging spots and non-discharging spots can be widened intentionally so as to prevent arc jumps.
In the discharge-lamp lighting device according to the present invention, it is preferable that the controlling portion sets the frequency of the drive-controlling signal variably corresponding to either the current value or the voltage value of the lamp. In this case, it is preferable that the controlling portion sets the frequency of the drive-controlling signal to be higher (e.g., 340 Hz) than a predetermined frequency range when the lamp current value is equal to or higher than a predetermined value or when the lamp voltage value is equal to or lower than a predetermined value. Alternatively, it is preferable that the controlling portion inhibits superimposition of the triangular wave signal onto the current-controlling signal when the lamp current value is equal to or higher than a predetermined value or when the lamp voltage value is equal to or lower than a predetermined value.
According to this configuration, arc jumps can be decreased and the life of the lamp can be prolonged by suppressing and accelerating growth of spots caused by the tungsten halide oxide as circumstances demand, since the reduction of tungsten halide oxide requires an appropriate range for temperatures and a certain period of time. Therefore, controlling the frequency of the commutator by using either the lamp current or voltage can serve to control intentionally the amount of reduction of the tungsten halide oxide, i.e., the growth of spots.
For example, when the lamp voltage is low and much lamp current flows, the surface temperature of the electrodes also will be raised in general so as to provide a condition to accelerate the growth of the spots that is caused by reduction of the tungsten halide oxide. In this case, by shifting the frequency for driving the commutator to a higher value, the time for reducing the tungsten halide oxide can be shortened to suppress growth of the spots. This can be attained also by inhibiting superimposition of the triangular wave signal onto the current-controlling signal. Thereby, losses at switching elements composing the commutator can be decreased to prevent thermal destruction of the switching elements.
On the contrary, when the lamp voltage is high and less lamp current flows, the surface temperature of the electrodes is low as well, and thus, the spots caused by reduction of the tungsten halide oxide will be difficult to grow. In this case, by shifting the frequency for driving the commutator to a lower value, the time for reducing the tungsten halide oxide can be prolonged to accelerate the spots"" growth. This can be attained also by resuming the superimposition of the triangular wave signal onto the current-controlling signal.
An additional effect provided is to decrease the lowering of the lamp voltage caused by the growth of spots that shortens the distance between the electrodes, which is obtained by suppressing excessive growth of the spots when the lamp voltage is low. Moreover, since discharging spots can be made to grow intensively when the lamp voltage is high, arc jumps can be prevented, the distance between electrodes is shortened due to the growth of spots, and thus the lamp voltage can be lowered intentionally. That is, since a change in the lamp voltage over time can be decreased, the arc jumps can be decreased and also, the lamp life can be prolonged.
In the discharge-lamp lighting device according to the present invention, it is also preferable that the controlling portion sets the frequency of the drive-controlling signal variably corresponding to the temperature of the high-pressure discharge lamp.
According to this configuration, the arc jumps can be decreased and a lamp life can be prolonged by suppressing and accelerating growth of spots caused by the tungsten halide oxide as circumstances demand, since the reduction of tungsten halide oxide requires an appropriate range for temperatures and a certain period of time.
Therefore, if the frequency of the commutator can be controlled depending on the lamp working temperature, it will be possible to control intentionally the amount of reduction of the tungsten halide oxide, i.e., the growth of spots. For example, when the lamp temperature is high, the surface temperature of the electrode is also raised in general so as to provide a condition to accelerate the growth of the spots that is caused by reduction of the tungsten halide oxide. In this case, by shifting the frequency for driving the commutator to a higher value, the time for reducing the tungsten halide oxide is shortened, thereby suppressing growth of the spots.
On the contrary, when the lamp temperature is low, the surface temperature of the electrode is low as well, and thus the spots caused by reduction of the tungsten halide oxide will be difficult to grow. In this case, the time for reducing the tungsten halide oxide is prolonged by shifting the frequency for driving the commutator to a lower value, thereby accelerating growth of the spots.
An additional effect provided is to decrease the lowering of the lamp voltage caused by the growth of spots that shortens the distance between the electrodes, which is obtained by suppressing excessive growth of the spots when the lamp temperature is high. Moreover, since discharging spots can be made to grow intensively when the lamp temperature is low, arc jumps can be prevented, the distance between electrodes is shortened due to the growth of spots, and thus the lamp voltage can be lowered intentionally. That is, since a change in the lamp voltage over time can be decreased, the arc jumps can be decreased and also, the lamp life can be prolonged.
In the discharge-lamp lighting device according to the present invention, it is preferable that the predetermined frequency range is from 100 Hz to 270 Hz, and the controlling portion adjusts the waveform so that the time for the polarity inversion of the current flowing in the high-pressure discharge lamp is at most 40 xcexcsec in a 80% section of the rated current.
According to this configuration, the arc jumps can be decreased by shortening time in which the temperature of the electrode surface is low during the polarity inversion and also by matching the returning point of the arc after the polarity inversion with the origin of the previous arc. The frequency range for the sharp-pointed projections (spots) to grow will be specified as follows. When the frequency of the drive-controlling signal becomes lower than the lower limit of 100 Hz, the spots are destroyed due to impact at the time of polarity inversion of the lamp current. When the frequency of the drive-controlling signal becomes higher than the upper limit of 270 Hz and when the lamp electrode used is made of tungsten, the time for reduction of the tungsten halide oxide is shortened and growth of the spots is suppressed. For this reason, the frequency for the spots to grow is set to a range of 100 Hz to 270 Hz.
It is preferable that the discharge-lamp lighting device is provided with a choke coil connected in series with the high-pressure discharge lamp and having an inductance value that is higher in a high frequency region than in a low frequency region.
According to this configuration, polarity inversion can be performed instantly by using positively a back electromotive force generated at the choke coil at the time of the polarity inversion. In general, for controlling the commutator that commutates a direct current to an alternating current, a dead time is provided so that a switching element at the high side and a switching element at the low side will not turn ON simultaneously.
This dead time ranges from several xcexcsec to tens of xcexcsec, since it is set by considering an on-delay time and a rise time, and also an off-delay time and a fall time of the switching elements. Since the capacity of the switching elements is increased for a case of a high-power type lamp, the dead time will be increased further. In such a case, the temperature of the electrodes is lowered excessively during the dead time, and the discharge arc shifts to other spots so as to cause arc jumps.
A choke coil is inserted in series with the lamp in order to generate a back electromotive force for the choke coil to maintain the magnetic flux at the moment that the lamp current is interrupted during the polarity inversion of the lamp current, so that a current flows in the inverse direction. By using this positively, the time for the polarity inversion can be shortened, which cannot be obtained by only controlling the commutator. In some discharge-lamp lighting devices, a choke coil such as an air-core type or an open-magnetic circuit type having a large inductance value at a high frequency region is used. However, a choke coil L1 used in the present invention is limited to a close-magnetic circuit type such as a toroidal type having a large inductance value in a low frequency region.
For achieving the above-identified objects, a first system according to the present invention using the above-described discharge-lamp lighting device according to the present invention is characterized in that it includes at least a cooling device for cooling the high-pressure discharge lamp, a brightness detector for detecting brightness of the high-pressure discharge lamp, and a controlling device for decreasing the cooling capacity of the cooling device when brightness fluctuation is detected by the brightness detector.
According to this configuration, since shapes or the like of the spots formed on the lamp electrode surface vary depending on the lamp temperature as well, when detecting the fluctuation in brightness of the lamp, the arc jumps can be decreased by decreasing the cooling capacity of the cooling device so as to accelerate the growth of the spots. The brightness detector can be omitted in case a correlation to cause arc jumps has been established between a working environmental temperature of an apparatus such as a liquid crystal projector equipped with a discharge lamp and a separate component temperature inside the apparatus.
For achieving the above-described objects, a second system according to the present invention using a discharge-lamp lighting device according to the present invention, is characterized in that it includes at least a cooling device for cooling a high-pressure discharge lamp, a temperature detector for detecting the external temperature of the system, and a controlling device for setting the cooling capacity of the cooling device at a predetermined value when the external temperature detected by the temperature detector becomes lower than a predetermined value.
According to this configuration, when a correlation to cause arc jumps is established between the working environmental temperature of the apparatus equipped with the discharge-lamp lighting device and a separate component temperature inside the apparatus, and when a brightness detector for detecting brightness fluctuation of the lamp is omitted, a cooling condition of a cooling device equipped to the exterior for cooling the lamp is set previously to a controlling device of an apparatus such as a liquid crystal projector equipped with the discharge-lamp lighting device. Accordingly, when the external temperature becomes lower than a predetermined value (e.g., 10xc2x0 C.), the cooling capacity of the cooling device is set to a predetermined condition in order to accelerate the growth of the spots, so that the arc jumps can be decreased.